In transducers of condenser microphones, also referred to as capacitor microphones or electrostatic microphones, or other electromechanical pressure sensing devices, a membrane or diaphragm may act as one plate of a sensor capacitor. Pressure changes lead to changes in the distance between the plates. For condenser microphones there are two types, depending on the method of extracting the audio signal from the transducer: DC-biased microphones, and Radio Frequency (RF) or high frequency (HF) condenser microphones. With a DC-biased microphone, the capacitor plates may be biased with a fixed electrical charge. The voltage maintained across the capacitor plates changes with the vibrations in the air. The sensor capacitance of the plates is inversely proportional to the distance between them for a parallel-plate capacitor. Within a time-frame of the sensor capacitance change the charge is practically constant and the voltage across the capacitor changes instantaneously to reflect the change in capacitance. The voltage across the capacitor varies above and below the bias voltage.
Subject to manufacturing tolerances, silicon (condenser) microphones and comparable pressure sensors, such as Micro-Electro-Mechanical Systems (MEMS), typically experience variances in the capacitive sensor's sensitivity, which are generally undesired and may be compensated by means of proper measures, such as appropriate amplifier settings and/or variation of the bias voltage, which may be applied between the diaphragm and a back-plate structure forming the sensor capacitor. For such compensation it is necessary to determine the sensor's sensitivity during manufacturing and/or in the field, for example, at power-on, periodically or continuously in the background.
For example, the sensitivity of microphones or pressure sensors may be calibrated by applying a reference signal, such as a reference pressure. However, this concept is relatively cumbersome with respect to time and/or measurement equipment. Hence, it is typically avoided for microphones.
A variation of sensitivity also reflects in a course or trend of the sensor capacitance versus the bias voltage. An electrostatic force associated with the bias voltage is nonlinear due to its inverse square relationship with the air gap thickness between the capacitor electrodes. This gives rise to a phenomenon known as ‘pull-in’ or collapse that reduces the dynamic range of the diaphragm displacement. If the bias voltage exceeds this pull-in or collapse limit, the diaphragm will collapse, i.e. stick to the back plate. Conventionally, the pull-in or collapse voltage has been used as the key figure for sensitivity. Thereby the collapse voltage denotes the bias voltage at which the sensor capacity increases rapidly.
The pull-in voltage may be determined by recording a measurement series of capacity values versus corresponding bias voltages. Naturally, the accuracy of pull-in voltage determination is dependent on the distance of adjacent measurement points. In practice, a high accuracy requires a relatively high amount of measurement points and a correspondingly long measurement/calibration time, as the range of where to find the pull-in voltage may be relatively large due to manufacturing tolerances.
A further conventional method to determine the pull-in voltage is to apply a ramp-like bias voltage to a series circuit of a sensor and a resistor. As long as the bias voltage is lower than the pull-in voltage the current through the resistor is relatively constant. However, as soon as the bias voltage exceeds the sensor's pull-in voltage its diaphragm collapses. The related rapid increase of the sensor's capacitance yields a current impulse which may be evaluated directly or in form of a voltage across the resistor. This concept has the disadvantage that mechanical time constants (e.g. mass of the diaphragm, spring constant, mechanical and/or acoustic attenuation, volume of the housing, etc.) and electrical time constants (e.g., increase of the bias voltage, sensor capacity, electrical resistance, etc.) influence the measurement and, hence, hamper an exact determination of the pull-in voltage.
Hence, it is desirable to provide an improved concept for determining the sensitivity of capacitive sensors, such as condenser microphones and comparable pressure sensors.